Crew is an activated ability that appears on Vehicles — artifacts that sit on the battlefield as noncreatures until you give them a pilot. A line like Crew 2 means: tap any number of creatures you control with total power 2 or more, and this Vehicle becomes an artifact creature until end of turn. Only total power matters, so one large creature or three small ones both get the job done, and the tapped creatures don't attack or deal damage — they simply crew.
Because it's an activated ability rather than an attack, you can crew at instant speed: after blockers are declared, or on an opponent's turn to create a surprise blocker. Creatures that are still summoning sick can crew too, since the tap is a cost of the Vehicle's ability, not of the creature. Vehicles let a deck turn spare bodies into real threats.
